The present invention relates in general to computer systems, and in particular to peripheral devices used in computer systems. Still more particularly, the present invention relates to a method and system for automatically generating printer profiles.
As computing moves into the multimedia era, the days in which color was an unneeded luxury and monochrome monitors were commonplace, have given way to color image processing being an integral feature used by most applications. As the commercial demand for color grew, so too did the complexity of the hardware and software which was designed in response to that demand. Many color image processing techniques have been developed to deal with the issues raised by the integration of color into the computing paradigm.
Color matching is one such technique, which is used when transferring color images and documents between color devices, such as monitors, scanners and printers. Color matching is a useful interface technique because color devices have different color capabilities, describe color in different terms and operate in different color spaces. For example, a color display monitor in a computer system may create and describe colors in terms of red, green and blue ("rgb") values, and is then said to work in the RGB color space. The rgb values for this display monitor are device dependent, meaning the rgb values are particular for that monitor or brand of monitor. Because the rgb values are device dependent, colors displayed on different monitors will probably not be visually identical even for the same rgb values.
Most printers create and describe colors in device dependent terms differing from monitors. Printers use, for example, cyan, magenta, yellow and black ("cmyk") values to describe colors, and are said to work in the CMYK color space. Again, because the cmyk values are device dependent, colors printed on any given printer will probably not match colors printed on a different printer for the same cmyk values.
Further complicating color matching between color devices is that different color devices have different color capabilities. Every color device, such as a scanner, printer, or monitor, has a range of colors that it can produce. This range of producible colors is known as a gamut. To exemplify the difficulties caused by differing device gamuts, consider monitors and printers. Those skilled in the art will recognize that color display monitors can produce and display hundreds to thousands of colors. Color printers, however, typically have a smaller number of printable colors. Consequently, in most situations the gamut for a color display monitor exceeds the gamut for a color printer. As a result some colors displayed on display monitors cannot be produced by color printers.
Color matching models translate colors between devices while trying to maintain the perceived color appearance. For example, suppose that a user creates an image on a monitor. If she or he prints this image without any color matching, the color appearance of the printed image will differ significantly from that of the original. Using a color matching model, this change can be reduced to a perceptionally acceptable level.
In addition to color matching models, device profiles are needed for each color device to be matched. These profiles describe the basic color characteristics of the device and include a minimum required set of parameters for the color matching model to use. By searching through the source and destination device profiles, a color matching model can translate the source colors into a device independent color space and from this space into the destination device color space. Therefore, every color device is provided with a device profile with the required information for at least a default color matching model, if a custom-designed color matching model is not being used. In order to provide the best results, this profile information needs to be optimized for the particular color matching model in use.
It has been found, however, that certain color matching models, such as the default color matching model shipped provided color devices produced by Apple Computer, Incorporated, located in Cupertino, Calif., are not well suited to converting from one color space to another, particularly with respect to printers, due to certain assumptions about the linearity of color transformations which are implicit in these models. These assumptions, and an exemplary default color matching model, are described in more detail in the detailed description that follows.
Printer profiles have been conventionally created by the manual measurement of printed test patches to produce colorant values and tone reproduction tables. Due to the weakness of some conventional models, these printer profiles have been manually adjusted to obtain the proper colorant matrices so that the color conversions are perceptionally acceptable. Since this manual adjustment is very time consuming (on the order of a week to set up a profile for a printer) and costly, it would be desirable to overcome the problems caused by these default models. Although one solution would be to create a better color matching model, many color devices have already been produced which operate using models which are integral with color conversion systems that would be commercially inviable to replace.